Changes to fit better with the needs of the download manager.

I thought that the client would only have clientIDs available, so I
created a new set of RemoveRequestByClientId functions.  However, it
turns out that the client won't be persisting these either, so I am
now removing them

They were added in https://codereview.chromium.org/2197573003/

Also, the "Get" functions now return full SavePageRequests instead
of only clientIds.

The intended usage pattern is for a client to make a request then
forget about it.  When the client wants to do something with a
request, it will first need to make a query using the
GetQueuedRequest API, then it will refer to specific requests by
request_id.

I also made RequestQueue::RemoveRequest into RemoveRequests, since
both the higher and lower level functions take a vector of request ids,
to better match the calling code.

BUG=610521

Review-Url: https://codereview.chromium.org/2228813003
Cr-Commit-Position: refs/heads/master@{#411533}
12 files changed